The invention relates to the use of whitened finely divided plastics for whitening paper coating slips based on synthetic binders, and paper coating slips which contain such whitened plastics.
Aqueous coating slips based on polymeric binders are used in very large amounts for the production of coated papers and boards. Customary binders for paper coating slips comprise, for example, butadiene/styrene, styrene/butyl acrylate, acrylonitrile/butadiene/styrene, styrene/butadiene/alkyl acrylate, alkyl acrylate, ethylene/vinyl chloride and ethylene/vinyl acetate copolymers and the homopolymers polyethylene, polyvinyl chloride, polyvinylidene chloride, polyvinyl acetate and polyaddition compounds such as polyurethanes.
In addition to the customary white pigmentsxe2x80x94especially china clay and calcium carbonatexe2x80x94the coating slips contain as a rule, for optical brightening, anionic whiteners, for example those of the bis-triazinyl-aminostilbene-disulphonic acid type. However, these whiteners lead only to very unsatisfactory whitening effects and to very low greying limits (=whitener concentration at which further addition of whitener produces no increase, or even a decrease, in the whiteness). In addition, the stated whitener types have insufficient lightfastness in the coating slips.
It has already been recommended that the whitening effect in coating slips can be improved by using hydrophilic cobinders or carriers (xe2x80x9cDas Papierxe2x80x9d 36 (1982) 66; German Offenlegungsschrift 3,502,038, EP-A 4 37 90). However, the water sensitivity of the coated paper is increased to an undesirable extent by the hydrophilic character of the components added.
It has also been proposed to use dispersion brighteners in the presence of dispersants (British Patent Specifications 1,294,273 and 1,349,934). The whiteness achievable and the strong dependence of the whiteness on the drying temperature may, however, be unsatisfactory.
Water-soluble whiteners have also already been condensed with aminoplasts and these condensates are recommended in particular for whitening paper coating slips (German Offenlegungsschrift 3,112,435). However, these proposals have not become established in practice, owing to insufficient lightfastness and rheological difficulties.
The production of coated papers and boards having a high degree of whiteness, a high greying limit and good lightfastness is a difficult problem which has not been solved satisfactorily to date.
It has now been found, surprisingly, that whitened finely divided plastics which are not aminoplasts impart very high levels of whiteness, high greying limits and high lightfastnesses to paper coating slips.
The invention therefore relates to the use of whitened finely divided plastics from the series comprising polyacrylonitrile, polyamide, polyester, polyolefins, vinylaromatic polymers and cellulose or regenerated celluloses for whitening paper coating slips based on synthetic binders.
The invention furthermore relates to paper coating slips whitened with the aid of these whitened plastics.
The plastics on which the whitened plastics to be used according to the invention are based comprise polyacrylonitriles, polyamides, such as, for example, polyamide 6 and polyamide 6.6, and polyesters, in particular those based on phthalic acid, isophthalic acid and/or terephthalic acid and ethylene glycol, 1,4-butanediol and/or bis(hydroxymethyl)-cyclohexane, polyesters based on maleic acid and/or fumaric acid and/or p-phenylenediacetic acid and/or 4,4xe2x80x2-diphenyldicarboxylic acid and/or 2,6-naphthalenedicarboxylic acid and ethylene glycol, polyesters based on 4-hydroxybenzoic acid, polyolefins, such as polyethylene and polypropylene, vinylaromatic polymers, such as polystyrene, cellulose and regenerated celluloses, such as viscose and acetylcellulose (in particular cellulose di- and triacetate).
Preferred polyacrylonitriles have intrinsic viscosities (measured in dimethylformamide at 20xc2x0 C.) of from 1.18 to 2.22 dl/g, preferred polyamides have relative viscosities (measured in a 1% strength by weight solution in m-cresol at 25xc2x0 C. using an Ubbelohde viscometer) of from 2 to 5.5, preferably from 2 to 4, and preferred polyesters have intrinsic viscosities (measured in phenol/tetrachloroethane=60:40 (parts by weight) at 25xc2x0 C.) of from 0.4 to 1.5 dl/g.
For the purposes of the invention, plastics powders having a particle size (determined as weight average) of less than 1 xcexcm are referred to as xe2x80x9cfinely dividedxe2x80x9d.
The person skilled in the art can base his choice of preferred whiteners on the optical brighteners which are used for whitening in the textile sector. On textile fibres, preferred whiteners give comparatively high lightfastnessesxe2x80x94measured in the Xenotest on the basis of the guidelines for the determination of colour fastnesses according to DIN 54004xe2x80x94of at least 4, preferably from 5 to 7.
Preferably used whiteners for finely divided polyacrylonitriles are compounds of the formulae: 
in which
R denotes ethyl or phenyl; 
in which
x represents CH or N,
R1 represents CH3 or CH2xe2x80x94C6H5 and
R2 represents H or SO2xe2x80x94CH3; and 
in which
R3 denotes NH2, CH3, NHxe2x80x94C3H6xe2x80x94N(CH3)3+Anxe2x88x92, C2H4xe2x80x94N(CH3)3+Anxe2x88x92, CH2xe2x80x94CH(CH3)xe2x80x94N(CH3)3+Anxe2x88x92, CH(CH3)xe2x80x94CH2xe2x80x94N(CH3)3+Anxe2x88x92, C2H4xe2x80x94Oxe2x80x94CH(CH3)xe2x80x94CH2xe2x80x94N(CH3)3+Anxe2x88x92 or C2H4xe2x80x94COxe2x80x94NHxe2x80x94C3H6xe2x80x94N(CH3)3+Anxe2x88x92 and
Anxe2x88x92 denotes the anion of a mineral acid, formic acid, acetic acid, lactic acid or xe2x88x92O3SOCH3.
Preferably used whiteners for finely divided polyamides are compounds of the formulae 
in which
M represents an alkali metal or hydrogen.
Preferably used whiteners for finely divided polyesters are compounds of the formulae 
in which
R4 denotes phenyl or a radical of the formula 
and
R5 denotes a radical of the formula 
in which
R6 and R7, independently of one another, denote hydrogen, C1-C4-alkyl or C1-C4-alkoxycarbonyl and
X1 represents xe2x80x94CHxe2x95x90CHxe2x80x94, 
in which
R8 and R9, independently of one another, denote hydrogen or C1-C4-alkyl,
R10 denotes C1-C4-alkyl, phenyl or C1-C4-alkoxycarbonyl
n denotes 0 or 1; 
in which
the CN groups may each be in the o-, p- or m-position, and their isomer mixtures; and 
in which
R11 denotes C1-C4-alkoxy-carbonyl.
Very particularly preferred whitened plastics contain whitened polyacrylonitrile with whiteners of the formulae (1) or (2), whitened polyamide with whiteners of the formula (5) or whitened polyesters with whiteners of the formulae (6) or (7).
The finely divided whitened plastics can be incorporated in powder form into the paper coating slips. In most cases, however, it is likely to be more convenient to disperse the finely divided plastics in the aqueous phase before or after the application of the whitener and, after the whitener has been applied if appropriate, to incorporate the aqueous plastics dispersion obtained into the paper coating slips.
These plastics dispersions may contain A) from 1 to 30% by weight, preferably from 5 to 25% by weight, of the whitened polymers, B) from 1 to 50% by weight, preferably from 5 to 20% by weight, of surface-active substances, C) from 0 to 15% by weight of preservatives and D) from 20 to 98% by weight, preferably from 55 to 90% by weight, of water, it being possible to replace up to half the water by hydrotropic substances, such as, for example, ethylene glycol or glycerol, the stated percentages being relative to the sum A+B+D in each case.
The surface-active substances used may be anionic, cationic and/or nonionic surface-active substances, as described, for example, in Methoden der Organischen Chemie [Methods of Organic Chemistry] (Houben-Weyl), 4th Edition, Vol. XIV/I, Georg Thieme Verlag, Stuttgart 1961, page 190 et seq., and in German Offenlegungsschrift 2,334,769, pages 8 to 10 ( British Patent Specification 1,417,071). Dispersions of the finely divided whitened plastics to be used according to the invention may be prepared, for example, as follows:
First, the pulverulent polymers are treated with the whiteners suitable for the selected substrate, in aqueous systems at temperatures of from 60xc2x0 C. to the boiling point, until the whiteners have been completely applied to the substrate. If appropriate, further auxiliaries are used.
The whiteners can, however, also be fixed on the corresponding polymer materials in the fusion process.
The amount of whiteners used on the substrate depends on the desired brightening effect; it is between 0.01 and 5% by weight of pure active substance, relative to the plastic (solid) used.
The whitened plastic powder isolated is then homogenized with vigorous stirring, after the addition of a surface-active substance and optionally water. Thereafter, the amount of surface-active substance can be increased, optionally to the total amount required for the stability of the dispersion. The suspension obtained is then precomminuted and wet-milled. The precomminution can be effected by means of stone mills or toothed colloid mills. The subsequent wet comminution can be effected in colloid mills, vibratory mills, conical mills and vibromills and in dissolvers or in sub-micro dispersers. However, continuous stirred ball mills containing grinding media, preferably those comprising SiO2 of from 0.2 to 5 mm diameter, are preferably used.
After the milling treatment, further amounts of surface-active substances or hydrotropic substances, preservatives, wetting agents, antifoams and/or water may also optionally be added, unless this has already been done at an early stage, for example before the milling.
The whitened plastics powder isolated can, however, also be incorporated directly into the paper coating slips after dry milling.
A particularly advantageous possibility for the preparation of a dispersion of whitened plastic is the combination of the whitener application process and the milling process:
For this purpose, the pulverulent plastic, together with the corresponding whitener, water and surface-active substances, is wet-comminuted as described above at temperatures from 60xc2x0 C. to the boiling point, optionally after homogenization and precomminution. Complete brightening takes place during the milling process in the milling apparatus, preferably in a continuous stirred ball mill containing SiO2 grinding media.
The amount of whiteners used in the paper coating slip depends on the desired whitening effect. In general, from 0.01 to 0.5% by weight of pure whiteners (relative to the solid of the paper coating slip to be brightened) is sufficient. A particular advantage is that, depending on the coating slip composition, the achievable greying limit is extremely high.
The paper coating slips have in general solids contents of from 35 to 80, preferably from 40 to 70,% weight. In addition to the whitened finely divided plastic to be used according to the invention, they contain in general (data relative in each case to solid)
a) from 100 to 150 parts by weight of inorganic pigment,
b) from 3 to 25 parts by weight of binder, of which up to a half optionally consists of natural (that is to say nonsynthetic) cobinder (such as, for example, starch, casein),
c) up to 1 part by weight of thickener and
d) up to 2 parts by weight of wet strength agent.
The stated percentages in the examples below relate in each case to weight; parts are parts by weight.